Raman and Infrared Structural Investigation of <i>x</i>Rb₂O·(1 − <i>x)</i>GeO₂ Glasses

Abstract

Infrared reflectance and Raman spectra of rubidium germanate glasses, x Rb 2 O·(1 − x )GeO 2, have been measured and studied as a function of Rb 2 O mole fraction in the range 0 ≤ x ≤ 0.60. The spectra of corresponding crystalline germanate phases, GeO 2 -quartz, GeO 2 -rutile, Rb 2 Ge 4 O 9, K 4 Ge 9 O 20, K 2 Ge 2 O 5, and K 2 GeO 3, have been also measured for a better understanding of the glass spectra. At very low content ( x ≤ 0.01) Rb 2 O was found to induce a redistribution of GeO 4 tetrahedra in rings smaller than those encountered in quartz- and glassy GeO 2 . The spectral manifestations of this trend are the frequency increase of the Raman active symmetric stretching vibration of Ge−O−Ge bridges and the parallel decrease of the infrared frequency of the asymmetric stretching of Ge−O−Ge bridges. Beyond the composition x = 0.01 and up to x ≈ 0.15−0.20, the predominant modification mechanism involves the transformation of GeO 4 tetrahedra into GeO 6 octahedra, with characteristic Raman scattering at 315, 590, and 630 cm -1 . At higher alkali content and up to the limits of glass formation, tetrahedral Q n species with nonbridging oxygens are formed ((4 − n ) is the number of nonbridging oxygens per tetrahedron). Thus, Q 3 appears beyond x = 0.10, giving place to Q 2 ( x > 0.30), Q 1 ( x > 0.50), and Q 0 ( x = 0.60). The characteristic Raman bands of these germanate tetrahedra were measured at 865, 765, 740, and 720 cm -1, respectively. Based on semiempirical models, the spectroscopic results were quantified to yield the average coordination number of Ge atoms, as well as the fraction of Ge atoms in 6-fold coordination. Both were found to exhibit a maximum value at approximately 15 mol % Rb 2 O.